The operation of most semiconductor devices is governed by the control of mobile charge carrier concentrations different from that generated at thermal equilibrium.
Prior to the present invention, only four general methods were known by which to control and modulate the concentration of excess or free (these two terms are used interchangeably throughout this discussion) charge carriers in solid state semiconductor devices. These four known methods will be described hereinbelow following a general discussion of those fundamental mechanisms of operation of semiconductor devices which are necessary in order to appreciate the advantages of the instant invention.
By way of explanation, in a perfect semiconductor lattice with no impurities or lattice defects--an intrinsic semiconductor--no charge carriers are present at zero Kelvin since the valence band is filled with electrons and the conduction band is empty. At higher temperatures, however, electron-hole pairs generated as valence band electrons are excited thermally across the band gap to the conduction band. These thermally generated electron-hole pairs are the only charge carriers present in an intrinsic semiconductor material. Of course, since the electrons and holes are created in pairs, the conduction band electron concentration (electrons per cubic centimeter) is equal to the concentration of holes in the valence band (holes per cubic centimeter). It is well known, but worth emphasizing, that if a steady state carrier concentration is to be maintained, there must be recombination of the charge carriers at the same rate that they are generated. Recombination occurs when an electron in the conduction band makes a transition to an empty state (hole) in the valence band, either directly or indirectly through the agency of a mid-gap recombination center, thus annihilating the pair.
In addition to thermally generated charge carriers, it is possible to create carriers in semiconductor materials by purposely introducing certain impurities into the crystal lattice. This process is called doping and represents a common method of varying the conductivity of semiconductors. By doping, a semiconductor material can be altered so that it has a predominance of either electrons or holes, i.e., it is either n-type or p-type. When a crystal lattice is doped such that the equilibrium carrier concentrations are different from the intrinsic carrier concentrations, the semiconductor material is said to be "extrinsic". When impurities or lattice defects are introduced into an otherwise perfect lattice crystal, additional levels are created in the energy band structure, usually within the band gap. For instance, the introduction of phosphorous in silicon or germanium, generates an energy level very near the conduction band. This new energy level is filled with electrons at zero Kelvin, and very little thermal energy is required to excite these electrons to the conduction band. Thus, at about 50-100 Kelvin, virtually all of the electrons in the impurity level are donated to the conduction band. Semiconductor material doped with donor impurities can have a considerable concentration of electrons in the conduction band, even when the temperature is too low for the intrinsic charge carrier concentration to be appreciable.
Now that the reader can appreciate the significance of the presence of excess charge carriers for electrical conductivity, it must be noted that these carriers can also be created by optical excitation or they can be injected across a forward biased p-n junction or a Schottky barrier. Simply stated and regardless of the manner in which the excess carriers are generated, they can dominate the electrical conduction processes in a semiconductor material. It has previously been stated that there are four known methods of modulating the concentration of free charge. Those four methods are described below:
(1) In 1948, Bardeen, Brattain, and Schockley ushered in the modem era of semiconductor electronics when they demonstrated the operation of a solid state amplifier by successfully modulating the flow of injected minority charge carriers in bipolar junction transistors. The bipolar junction transistor is a three terminal device in which the flow of current through two terminals can be controlled by small changes in the current at the third terminal. This control feature provides for the amplification of small signals or for the switching of the device from an "on" state to an "off" state. In other words, the bipolar transistor is employed to modulate the injection and collection of minority charge carriers across a semiconductor junction. More particularly, and considering, for instance, in a p-n-p bipolar structure (the operation of an n-p-n bipolar structure is simply the reverse of the operation of the p-n-p structure), the negative side of the forward biased junction is the same as the negative side of the reverse biased junction. With this configuration, the injection of holes from the p-n junction into the center n region supplies the minority carriers, holes, to participate in the reverse flow of current through the n-p junction. As should now be evident, the designation of this device as "bipolar" relates to the critical importance of the action of both electrons and holes.
In operation, the reverse saturation current through the p-n junction of the device depends upon the rate at which minority carriers are generated in the neighborhood of the junction. It is possible to increase the reverse current through the junction by increasing the rate of electron-hole pair generation. This can be accomplished with light (as discussed below with respect to photodetectors). Electrically, a convenient hole injection device is a forward biased p-n junction in which the current is due primarily to holes injected from the p region into the n material. If the n side of the forward biased junction is the same as the n side of the reverse biased junction, the resultant p-n-p structure operates when the injection of holes from the p-n junction into the center n region supplies minority carrier holes to participate in the reverse current flow through the n-p junction of the transistor. Of course, the n-region is narrowed so that the injected holes do not recombine in the n region (the base of this p-n-p bipolar transistor) before they can diffuse to the depletion layer of the reverse-biased junction.
Finally, when used as a switch, this type of transistor is usually controlled in two conduction states, referred to as the "on" state and the "off" state. While transistors do not function as a short circuit when turned on and as an open circuit when turned off, they are able to approximate these actions. In transistor switching, the emitter junction is forward biased and the collector is reverse biased, with a reasonable mount of current flowing out of the base. If the base current is switched to zero, the collector current will be negligible. This is the "off" state. However, if the base current is positive and sufficiently large, the device is driven to the saturation regime and the transistor is in its "on" state. Therefore, in the typical switching operation, the base current swings from positive to negative, thereby driving the device from saturation to cutoff and vice versa.
(2) The second conventional method of controlling the concentration of free charge carriers is implemented by metal-oxide-semiconductor field effect transistor (MOSFET) devices. By way of background, one of the most widely employed electronic devices, particularly in digital integrated circuits, is the metal-insulator-semiconductor (MIS) transistor. In an MIS transistor, the concentration of charge carriers in the conduction channel is controlled by a voltage applied at a gate electrode isolated from the channel by an insulator. The resulting device may be referred to generically as an insulated-gate field effect transistor (IGFET). However, since most IGFETs are made using a metal (typically aluminum) for the gate electrode, silicon-dioxide as the insulator, and silicon as the semiconductor material, the term MOS field effect transistor or MOSFET is commonly used.
In operation of a MOSFET, consider an n-type channel formed on a p-type silicon substrate. The n-type source and drain regions are formed by diffusing or implanting dopant atoms into a lightly doped p-type substrate. A thin oxide layer separates the metal gate from the silicon surface. No current flows from the drain to the source unless there is a conducting n-channel between them, since the drain-substrate-source combination includes oppositely directed p-n junctions disposed in series. When a positive voltage is applied to the gate relative to the substrate (the source in this example), positive charge carriers are deposited on the gate metal. As a result of this deposition, negative charge carriers are induced in the underlying silicon by the formation of a depletion region. In addition, a thin surface region containing mobile electrons is formed. The induced electrons form the channel of the FET and allow current to flow from the drain to the source. The effect of the gate voltage is to vary the conductance of the induced channel for low drain-to-source voltage. The MOS field effect transistor is particularly useful in digital circuits, in which it is switched from the "off" state (no conducting channel) to the "on" state. Both n-channel and p-channel MOS transistors are in very common usage.
The MOS structure can be thought of as a capacitor in which one plate is a semiconductor. If a negative voltage is applied between the metal and the semiconductor, a negative charge is effectively deposited on the metal. In response thereto, an equal net positive charge is accumulated at the surface of the semiconductor. In the case of a p-type substrate, this occurs by hole accumulation at the semiconductor-oxide interface. Since the applied negative voltage depresses the electrostatic potential of the metal relative to the semiconductor, the electron energies are raised in the metal relative to the semiconductor. The energy bands of the semiconductor bend near the interface to accommodate the accumulation of holes. Because no current passes through the MOS structure, there is no variation in the Fermi level position within the bulk of the semiconductor. The result is a bending of the semiconductor bands near the interface so that the Fermi level is closer to the valence band adjacent the interface, thereby indicating a larger hole concentration than that arising from the doping of the p-type semiconductor material.
When a positive voltage is applied from the metal to the semiconductor, the potential of the metal increases, thereby lowering the metal Fermi level relative to its equilibrium position. As a result, the oxide conduction band is again tilted. The positive voltage deposits positive charge on the metal and effectively calls for a corresponding net negative charge at the surface of the semiconductor. Such a negative charge in p-type material arises from depletion of holes from the region near the surface which leaves behind uncompensated ionized acceptors. In the depleted region, the hole concentration decreases, bending the bands down near the semiconductor surface. If the positive charge continues to increase, the bands at the semiconductor surface bend down still further. In fact, a sufficiently large voltage can cause a large electron concentration in the conduction band. The region near the semiconductor in this case has conduction properties typical of n-type material. This n-type surface layer is formed not by doping, but by "inversion" of what was originally p-type semiconductor material due to the applied voltage. This inverted layer, separated from the underlying p-type material by a depletion region, is the key to MOS transistor operation.
(3) The third known method of controlling the concentration of free charge carriers is by the photogeneration of free charge carriers of both polarities. This photogeneration of free charge carriers takes place in such state-of-the-art devices as photovoltaic cells, photoresistors, photodetectors and electrophotographic drums.
In general, when excess electrons or holes are created in a semiconductor material, there is a corresponding increase in the electrical conductivity of the material. In the event that the excess charge carriers are generated from optical excitation, the resulting increase in conductivity is called "photoconductivity". When photons are directed to impinge upon a semiconductor material, those photons having energies greater than the band gap energy are absorbed and electron hole pairs generated. The electron and hole created by this absorption process are excess carriers; since they are out of balance with their environment and exist in their respective bands, they contribute to the electrical conductivity of the material.
(4) The fourth known method of modulating the free charge carrier concentration in semiconductor materials is by controlling the physical structure of chalcogenide phase change materials as they undergo reversible amorphous to crystalline phase transformations. A detailed explanation of this phenomena was reported in the early work on optical and electrical Ovonic phase change materials pioneered by S. R. Ovshinsky at Energy Conversion Devices, Inc. These materials and technology are discussed in detail below.
Since the present invention has significant scientific applicability to and immediate commercial impact on many different segments of the electronic and semiconductor industries, said invention is discussed hereinbelow in three different, but related sub-sections. More particularly, the relevance of the instant invention is discussed with respect to: (A) semiconductor devices per se; (B) optically operable, fast, non-volatile phase change memories; and (C) electrically erasable, directly overwritable, multilevel single-cell memories.
A. Semiconductor Devices PA0 B. Early Optical Phase Change Memory PA0 C. Early Electrical Phase Change Memory
The operation of bipolar transistors and MOSFETs is summarized in general terms in sections (1) and (2), above. From this discussion, it is easy for one skilled in the art to recognize that both of these electronic devices operate as three terminal structures in which the concentration of free charge carriers present in the semiconductor material of the device can be electrically modulated for as long as the external field or externally applied base current is applied; however, upon termination of the external electrical modulating signal, the modulation terminates and the semiconductor material from which the device is fabricated reverts back to the concentration of free charge which is exclusively determined by the pre-existing nature of the material as modified by dopant elements incorporated into the host matrix of the material. However, charge can be injected and stored on the gate, which charge remains after removal of the gate voltage. It is noteworthy that these devices require three terminals and, in order to operate, must be doped so as to form n-conductivity and p-conductivity regions and/or channels. In contrast thereto, devices fabricated from the materials of the instant invention require only two terminals and are made from a homogeneous body of semiconductor material.
Non-ablative, optical state changeable data storage systems record information in a state changeable material that is switchable between at least two detectable states through the input of an optical energy signal of a given time and amplitude.
State changeable data storage material is incorporated in a data storage device having a structure such that the data storage material is supported on a substrate and encapsulated. In the case of optical data storage devices, the encapsulations include, for example, anti-ablation materials and layers, thermal insulation materials and layers, anti-reflection materials and layers, reflective layers, and chemical isolation layers. Moreover, various layers may perform more than one of these functions. For example, anti-reflection layers may also be anti-ablation layers and thermal insulating layers. The thicknesses of the layers, including the layer adapted to minimize the amount of energy necessary for effecting a state change and optimize the high contrast ratio, are optimized to obtain high carrier to noise ratio and high stability of the state changeable data storage materials.
A "state changeable" material is a material that can be switched from one detectable state to another detectable state or states when radiation of a given energy is projected onto the material. State changeable materials are specifically tailored so that the detectable states differ in their morphology, surface topography, relative degree of order, relative degree of disorder, electrical properties, optical properties including indices of refraction and reflectivity, or combinations of one or more of these properties. The state of a state changeable material is detectable by the electrical conductivity, electrical resistivity, optical transmissivity, optical absorption, optical refraction, optical reflectivity, or a combination of these inherent properties. A common procedure employed in currently available write once and erasable so-called "phase change" optical memory materials utilizes structural changes in the material between crystalline and amorphous phases to alter the free carrier density of the material. The change in free carrier concentration results in changes in the refractive index of the material which gives rise to changes in optical reflectivity.
The methods of formation of data storage devices using optically addressable state changeable material includes deposition of the individual layers, for example by evaporative deposition, chemical vapor deposition, and/or plasma deposition. As used herein, plasma deposition includes sputtering, glow discharge, and plasma assisted chemical vapor deposition.
Tellurium based semiconductor materials have been utilized as state changeable materials for data storage where the state change is a structural change evidenced by a change in reflectivity. This effect is described, for example, by Feinleib, deNeufville, Moss, and Ovshinsky, in "Rapid Reversible Light-Induced Crystallization of Amorphous Semiconductors," 18(6) Appl. Phys. Lett. 254-257 (Mar. 15, 1971); and in U.S. Pat. No. 3,530,441 to Ovshinsky for METHOD AND APPARATUS FOR STORING AND RETRIEVING INFORMATION. A description of tellurium-germanium-tin semiconductor systems without oxygen is contained in the article by Chen, Rubin, Marclio, Gerber, and Jipson, "Reversibility and Stability of Tellurium Alloys for Optical Data Storage,"46(8) Appl. Phys. Lett. 734-736 (Apr. 15, 1985). A description of tellurium-germanium-tin semiconductor systems with oxygen is contained in the article by Takenaga, Yamada, Ohara, Nishikiuchi, Nagashima, Kashibara, Nakamura, and Yamashita, "New Optical Erasable Medium Using Tellurium Suboxide Thin Film," Proceedings, SPIE Conference on Optical Data Storage 173-177 (1983).
Tellurium based state changeable semiconductor materials can be single or multi-phase systems. In these systems, the ordering phenomena include a nucleation and growth process (including homogeneous and/or heterogeneous nucleation) to convert a system of disordered materials to a system of ordered and disordered materials. Also in these systems, the vitrification phenomenon includes melting and rapid quenching of the phase changeable material to transform a system of disordered and ordered materials to a system of largely disordered materials. Such phase changes and separations occur over relatively small distances, with intimate interlocking of the phases and gross structural discrimination and are highly sensitive to local variation in stoichiometry.
A serious limitation of the rate of data storage is the slow ordering or erasing time. One aspect of the slow ordering time is the sensitivity of the ordering time to variables such as the manufacturing history and the service history (the order-disorder cycle history). For example, in order to attain high ordering speeds, on the order of 0.1 to 10 microseconds, it is sometimes necessary to age the device or to introduce seed crystals and/or nucleation sites after deposition of the chalcogenide state change layer, prior to the deposition of the subsequent layer (barrier and encapsulation layers). Another aspect of this problem is that switching or ordering time can increase with increasing order-disorder cycles (this increase is identified by increasing order in the disordered material or portions of the disordered material).
A further aspect of this problem is the time required for the erase-rewrite cycle. A previous solution to this was a two laser erase-write cycle. With the first laser, an entire track, data segment, or data segment sector would be erased (crystallized). With the second laser, the entire track, data segment, or data segment sector would be written (by programmed vitrification).
More rapid data storage is described in U.S. Pat. No. 4,876,667 to Ross, Bjomard, and Strand, DATA STORAGE DEVICE HAVING A PHASE CHANGE MEMORY MEDIUM REVERSIBLE BY DIRECT OVERWRITE, filed Jun. 22, 1987. This patent describes a data storage device using a chalcogenide semiconductor material or mixture of chalcogenide materials as a data storage medium. These materials have crystallization times of under 1 microsecond (1000 nanoseconds), direct overwrite capability, and a long cycle life; are miscible solid solutions of a telluride and a selenide, such as arsenic telluride-arsenic selenide, antimony telluride-antimony selenide, or bismuth telluride-bismuth selenide; have a sufficiently low crystallization temperature and a sufficiently fast crystallization time to be easily switched from a less ordered detectable state to a more ordered detectable state with solid state lasers; and have a sufficiently high crystallization temperature to provide a measure of archival thermal stability. The '667 patent describes crystallization temperatures for these materials as greater than about 120.degree. C. up to around 200.degree. C.; the switching time, i.e. the erase or crystallization time, is described as less than 1 nanosecond and preferably less than 300 nanoseconds.
While the optically addressable phase change materials described above represented an improvement over earlier materials, these optical materials (i) are still relatively slow (microsecond switching is slow by present standards of electronic memory devices); (ii) require relatively high energy inputs to initiate a detectable change in local order, (iii) have only analned a cycle life in commercial production on the order of about 100,000 cycles; and (iv) have a relatively high cost per megabyte of stored information compared to hard disk storage. Overcoming all of the foregoing deficiencies are objectives of the present invention.
The relatively slow switching speeds in known optically addressable phase change memories result from the fact that the memory media of the prior art have relied upon amorphous to crystalline phase transitions in order to obtain a large enough difference to be readily optically detectable in actual practice, that is phase transitions that provide for a reflectivity change of about 35%.
Since an amorphous-to-crystalline phase transition is necessary in order to produce this detectable difference, high energy inputs are required. It is these high energy inputs which are responsible for the relatively low cycle life of memory media of the prior art. While the chalcogenide material, which forms the active, light absorbing layer of the system, has no trouble absorbing the requisite amount of input energy, this energy has an adverse effect on the encapsulating layers of polymeric material which possess a different coefficient of thermal expansion vis-a-vis said chalcogenide material. It has been determined that cycling said memory media through tens of thousands of read-write cycles, creates stresses which eventually deform the polymer and prevent proper focusing of the input laser beam onto the discrete cells of the memory material.
Thus, an optically modulatable memory material that could switch between at least two detectable states that were closer together would yield much faster switching speeds, speeds in the nanosecond switching range. Further, this switching would require much lower energy, thereby avoiding deformation of the encapsulant and providing markedly improved cycle life.
The general concept of utilizing electrically writable and erasable phase change materials (i.e., materials which can be electrically switched between generally amorphous and generally crystalline states) for electronic memory applications is well known in the art and as is disclosed, for example, in U.S. Pat. No. 3,271,591 to Ovshinsky, et at., issued Sep. 6, 1966 and in U.S. Pat. No. 3,530,441 to Ovshinsky, et at., issued Sep. 22, 1970, both assigned to the same assignee as the present invention, and both disclosures of which are incorporated herein by reference (hereinafter the "Ovshinsky patents").
As disclosed in the Ovshinsky patents, such phase change materials can be electrically switched between two different structural states of generally amorphous and generally crystalline local order or between different detectable states of local order across the entire spectrum between the completely amorphous and the completely crystalline states. That is, the Ovshinsky patents describe that the electrical switching of such materials is not required to take place between completely amorphous and completely crystalline states but rather can be in incremental steps reflecting changes of local order to provide a "gray scale" represented by a multiplicity of conditions of local order spanning the spectrum between the completely amorphous and the completely crystalline states. The early materials described by the Ovshinsky patents could also be switched between only two structural states of generally amorphous and generally crystalline local order to accommodate the storage and retrieval of binary encoded information.
The electrically erasable phase change memories described in the Ovshinsky patents were utilized in a number of commercially significant applications. Subsequent developments in other fields of solid state, electronic memories and in other types of memories in general, such as those utilizing magnetic and optical media eventually displaced these early electrically erasable phase change technology in the marketplace and prevented these phase change electrical memories from being used in, for instance, personal computers.
In a typical personal computer there often arc four tiers of memory. Archival information is stored in inexpensive, slow, high storage capacity, non-volatile devices such as magnetic tape and floppy disks. This information is transferred, as needed, to faster and more expensive, but still non-volatile, hard disk memories. Information from the hard disks is transferred, in turn, to the still more expensive, faster, volatile system memory which uses semiconductor dynamic RAM (DRAM) devices. Very fast computers even transfer forth and back small portions of the information stored in DRAM to even faster and even more expensive volatile static RAM (SRAM) devices so that the microprocessor will not be slowed down in the computations by the time required to fetch data from the relatively slower DRAM. Transfer of information among the tiers of the memory hierarchy occupies some of the computer's power and this need for "overhead" reduces performance and results in additional complexity in the computer's architecture. The current use of the hierarchal structure, however, is dictated by the price and performance of available memory devices and the need to optimize computer performance while minimizing cost.
The electrically and optically erasable phase change memories described in the Ovshinsky patents had a number of limitations that prevented their widespread use as a direct and universal replacement for present computer memory applications, such as tape, optical, hard disk drive, solid state disk flash, DRAM, SRAM, and socket flash memory. Specifically, the following represent the most significant of these limitations: (i) relatively slow Coy present standards) electrical switching speed, particularly when switched in the direction of greater local order (in the direction of increasing crystallization); (ii) relatively high energy inputs are necessary in order to initiate a detectable change in local order; and (iii) a relatively high cost per megabyte of stored information, particularly in comparison to hard disk storage.
The most significant of these limitations is the relatively high energy input required to obtain detectable changes in the chemical and/or electronic bonding configurations of the chalcogenide material in order to initiate a detectable change in local order. Also significant were the switching times of the electrical memory materials described in the Ovshinsky patents. These materials typically required times in the range of a few milliseconds for the set time (the time required to switch the material from the amorphous state to the crystalline state); and approximately a microsecond for the reset time (the time required to switch the material from the crystalline state back to the amorphous state). The electrical energy required to switch these materials typically measured in the range of about a microjoule.
It should be noted that this amount of energy must be delivered to each of the memory elements in the solid state matrix of rows and columns of memory cells. Such high energy levels translate into high current carrying requirements for the address lines and for the cell isolation/address device associated with each discrete memory element. Taking into consideration these energy requirements, the choices of memory cell isolation elements for one skilled in the art would be limited to very large single crystal diode or transistor isolation devices, which would make the use of micron scale lithography and hence a high packing density of memory elements impossible. Thus, the low bit densities of matrix arrays made from this material would restfit in a high cost per megabyte of stored information.
By effectively narrowing the distinction in price and performance between archival, non-volatile mass memory and fast, volatile system memory, the memory elements of the present invention have the capability of allowing for the creation of a novel, non-hierarchal "universal memory system". Essentially all of the memory in the system can be low cost, archival and fast. As compared to original Ovshinsky-type phase change electrical memories, the memory materials described herein provide over six orders of magnitude faster programming time (less than 10 nanoseconds) and use extraordinarily low programming energy (less than 50 picojoules) with demonstrated long term stability and cyclability (in excess of 20 million cycles). Also, experimental results indicate that additional reductions in element size can increase switching speeds and cycle life.
The concept of utilizing Ovshinsky-type electrical phase change materials in non-erasable or non-reversible, write-once electrically programmable memories is described, for example, in U.S. Pat. No. 4,499,557 of Holmberg, et al. issued Feb. 12, 1985 and U.S. Pat. No. 4,599,705 of Holmberg, et al. issued Jul. 8, 1986, both of which are assigned to the same assignee as the present invention and the disclosure of which are incorporated herein by reference. These patents describe having tetrahedral chemical bonds, such as carbon, silicon, and germanium and alloys thereof as phase change materials that are utilized in a non-reversible or non-resettable mode. Such materials are disclosed as having, for example, characteristics which require threshold setting voltages of up to 10 volts, currents up to 25 milliamps and setting times of up to 100 microseconds. Thus, the set power required is up to 250 milliwatts with corresponding set times of up to 100 microseconds. These high voltages, amperages and time periods are not surprising when considered in light of the findings described in the instant specification. The fact is that these early devices relied on amorphous-to-crystalline phase transitions and therefore required relatively high energy inputs in order to obtain detectible output signals.
In general, development and optimization of the general class of chalcogenide memory materials has not proceeded at the same rate as other types of solid state electrical memories that now have substantially faster switching times and substantially lower set and reset energies. These other forms of memories typically employ several solid state microelectronic circuit elements for each memory bit, as many as three or four transistors per bit, for example, in some memory applications. The primary non-volatile memory elements in such solid state memories, such as EEPROM, are typically floating gate field effect transistor devices which have limited re-programmability and which hold a charge on the gate of a field effect transistor to store each memory bit. Since this charge can leak off with the passage of time, the storage of information is not truly non-volatile as it is in the phase change media of the prior art where information is stored through changes in the actual atomic configuration or electronic structure of the chalcogenide material from which the elements are fabricated. These other forms of memories now enjoy some limited acceptance in the marketplace.
In contrast to DRAM and SRAM volatile memory devices and unlike other non-volatile EEPROM devices, such as floating gate structures, no field effect transistor devices are required in the electrical memory devices of the present invention. In fact the electrically erasable, directly overwritable memory elements of the present invention represent the simplest possible electrical memory device to fabricate, comprising only two electrical contacts to a monolithic body of thin film chalcogenide material and a semiconductor diode for isolation. As a result, very little chip "real estate" is required to store a bit of information, thereby providing for a configuration of inherently high density memory chips. Further, and as described below, additional increases in information density can be accomplished in the memory elements of the present invention through the use of multibit storage in each discrete memory cell.
The solid state, electronic memories presently in use are relatively expensive to manufacture, the cost being typically about twice the cost per bit of storage capacity in relation to magnetic disk storage. On the other hand, these solid state, electronic memories provide certain advantages over magnetic disk memories in that they have no moving parts, require much less electrical energy to operate, are easy to transport and store, and are more versatile and adaptable for use with portable computers and other portable electronic devices. As a matter of fact, hard drive manufacturers are forecasting rapid growth in the use of ever smaller hard drives and eventually solid state memory storage in the portable computer field. In addition, these solid state memories are usually true random access systems as opposed to disk types which require physical movement of the disk head to the proper data ack for accessing the desired memory location.
However, in spite of such advantages, the higher cost of solid state electrically erasable memories have prevented them from enjoying a substantial share of the market now dominated by magnetic disk type memory systems. Although solid state electrically erasable memories could potentially be manufactured at reduced cost, the overall performance parameters of these materials are inadequate for them to fully replace magnetic disk systems.
We previously mentioned that there were only four known types of semiconductor devices which could be employed to modulate the concentration of free charge. Each of those devices were then discussed in some detail. A fifth semiconductor device which can be set to a plurality of different resistance values by relatively low energy pulses and which is capable of relatively fast switching characteristics will now be discussed in detail. After carefully perusing the following paragraphs describing the performance characteristics and the physics behind the operation of the device, the reader will understand why it was not categorized as a fifth type of charge concentration modulating semiconductor device.
A recently developed memory device is the metal-amorphous silicon-metal (MSM) electrical memory switch. See Rose, et at, "Amorphous Silicon Analogue Memory Devices", Journal of Non-Crystalline Solids., 115(1989), pp.168-70 and Hajto, et al, "Quantized Electron Transport in Amorphous-Silicon Memory Structures", Physical Review Letters, Vol.66, No. 14, Apr. 8, 1991, pp. 1918-21. This MSM switch is fabricated by the deposition of specifically selected metallic contacts on either side of a p-type amorphous silicon (a-Si) thin film. The importance of the selection of the metallic contact materials will be discussed later. MSM memory switches were disclosed in said publications as exhibiting relatively fast (10-100 ns) analogue switching behavior for voltage pulses of from 1-5 volts, thereby providing a range of resistances of from about 10.sup.3 to about 10.sup.6 ohms to which they can be set in a non-volatile manner. As should be readily apparent to skilled practitioners in the art, the MSM memory switches of Rose, et al and Hajto, et al, although exhibiting electrical switching characteristics (i.e.,times, energies and resultant device resistance) similar to the electrical switching characteristics of the memory elements of the instant invention, actually present significant operational differences.
The most significant electrical switching difference (relative to the switches of the instant invention) resides in the inability of the MSM memory switches to be directly overwritten. That is, the MSM switches cannot be modulated directly and bidirectionally from any one resistance in the analogue range of resistances to any other resistance in that range without first being erased (set to a specific starting resistance or "starting state"). More specifically, the MSM switch must first be set to the high resistance state (erased) before said switch can be set to another resistance value within the analogue range. In contrast-thereto, the memory elements of the instant invention do not require erasure before being set to another resistance in the range; i.e., they are directly overwritable.
Another significant difference in the electrical switching characteristics which exists between the MSM memory switches of Rose, et al and Hajto, et al and the electrical memory elements of the present invention resides in the bipolar behavior of the said switches. As is disclosed by Rose, et al, the MSM switches must be erased using electrical pulses of reverse polarity from those pulses used to write. Significantly, this reversal of polarity of the applied pulse is not required by the memory elements of the present invention, whether the instant memory elements are used for digital or analogue switching.
These differences in electrical switching characteristics between the MSM switches and the memory elements of the present invention are attributable to more than just a mere difference in the material from which the elements are constructed. These differences are indicative of the fundamental differences in switching mechanisms which characterize the physics of operation of the two devices. As alluded to above and as disclosed in the aforementioned articles, the electrical switching characteristics of the MSM memory switches are critically dependent upon the particular metal(s) from which the contacts are fabricated. This is because these MSM switches require a very highly energetic "forming" process in which metal from at least one of the contacts is transported into and formed as an integral portion of the switch body. In this process, a plurality (at least 15 from FIG. 1 of the Rose, et al paper) of progressively increasing 300 nanosecond, 5-15 volt pulses are employed to form the switch. Rose, et al state: ". . . X-ray microanalysis studies of the devices have been carried out, and the top electrode material has been found embedded in a filamentary region of the a-Si. This suggests that the top metal becomes distributed in the filament, and may play a role in the mechanism of switching . . . ." Rose, et al also specifically find that the dynamic range of the available resistances is determined by the metal from which the upper electrode contact is fabricated. As is stated by Rose, et al: ". . . it is found that its value is entirely (sic) dependent on the top contact, and completely independent of the bottom metallisation (sic), i.e. Cr top electrode devices are always digital and V top electrode devices are always analogue irrespective of the bottom electrode . . . "
It is within this metallic filamentary region where the electrical switching occurs; and without this mass migration of metal into the a-Si, there would be no switching, see the Haito, et al paper. In complete contradistinction thereto, the memory elements of the present invention do not require migration of the contact material into the thin-film memory element to achieve high speed, low energy, analogue, direct overwrite, memory switching. As a matter of fact, in the fabrication of the memory elements of the instant invention, great care is taken to prevent the diffusion of metal from either of the electrodes into the active chalcogenide material. In one embodiment of the device described in the instant invention, the electrodes are each fabricated as bilayered structures in which, for instance, carbon forms a thin film barrier to prevent migration or diffusion of, for instance, molybdenum into the chalcogenide switching material.
From the foregoing analysis of Rose, et al and Hajto, et al, it should be clear that MSM memory switches do not, by any stretch of the imagination qualify as a modulator of free charge concentration. Rather, MSM memory switches simply rely upon the creation of a filamentary metallic pathway through the amorphous silicon material in order to obtain a range of resistivities in much the same way as a modulated switch is used to control the flow of electrical current. A percolation pathway is established, the diameter of which can be increased or decreased to change the resistivity thereof. The filamentary pathway cannot be homogeneous. No movement of Fermi level position is involved in the switching process. No change in the activation energy of the semiconductor material need be invoked to explain the operation. No atomic scale movement of lone pairs of non-bonding electrons is present. Crystallite size and surface to volume ratio thereof is not important. But most importantly, it is impossible for Rose, et al and Hajto, et al to directly overwrite information stored in the cells of their memory material. The MSM switch requires stored information to be erased before new information can be written. It is not surprising that-Rose, et al have asserted that their MSM switch is limited to one million cycles while the memory elements of the instant invention were cycled over 20 million cycles without prior to ending the test.
Simply stated, no solid state memory system developed prior to the present invention, regardless of the materials from which it was fabricated, has been inexpensive; easily manufacturable; electrically writable and directly erasable (overwfitable) using low input energies; capable of multibit storage in a single cell possesses gray scale capabilities), non-volatile; and capable of very high packing density. It is submitted that the memory system described below, because it addresses all of the deficiencies of known memory systems, will find immediate widespread use as a universal replacement for virtually all types of computer memory concurrently in the marketplace. Further, because the memories of the present invention can be fabricated in an all thin-film format, three-dimensional arrays are possible for high speed, high density neural network, and artificial intelligence applications. The memory system of the present invention is therefore uniquely applicable to neural networks and artificial intelligence systems because its multi-layer, three-dimensional arrays provide massive mounts of information storage that is rapidly addressable thus permitting learning from stored information.
It is clear from the discussion above that the quantitative changes in switching speed and energy requirements of the memories of the present invention compared to the phase change memories of the prior art demonstrates that the memory materials of the present invention define an entirely new class of modulatable semiconductor material from which to fabricate devices such as memory materials. In addition, the prior art has no analog to the direct overwrite, wide dynamic range and multibit storage capabilities of the present invention. Further, the operation of the semiconductor materials of the present invention occurs solely in the crystalline state and is therefore vastly different from the operation of all prior an optical and electrical memory materials which have relied upon crystalline-to-amorphous phase transitions. Moreover, that difference in operation of devices fabricated from the semiconductor materials of the instant invention represents a direct and fundamental consequence of the manner in which not only the concentration of free charge can be modulated, but the fact that the new concentration of free charge to which the device has been modulated remains constant after that electric field has been removed. This feature represents a fifth and fundamentally new mechanism for modulating the concentration of free charge in semiconductor devices and makes possible a range of new and simple switching and amplification techniques with the capability of significant impact upon the semiconductor industry.